(1) Field of the Invention
The present invention relates to a process for producing optical waveguide provided substrates each having a ridge shaped optical waveguide portion to be favorably used as a second harmonic generation element of a quasi phase matched type and optical modulator.
(2) Related Art Statement
The so called ridge shaped optical guidewaves have been expected as optical modulators, optical switching elements, etc. Quasi-phase matched (QPM) type second harmonic generation (SHG) devices using optical waveguides in which a periodically reversibly polarized structure is formed in a single crystal made of lithium niobate or lithium tantalate have been expected as blue laser light sources to be used for light picks. The second harmonic generation devices can find wide uses such as optical disc memorization, medical uses, optochemical uses, various optical measurements, etc.
Heretofore, in order to form a ridge shaped structure, it is a common knowledge that a mask pattern is transferred upon a substrate by photolithographic techniques, and the remainder of the substrate other than the mask pattern is removed, for example, by reactive ion etching (RIE process). In the case of an optical modulator in which an intensity, phase, wavelength or the like of light is modulated, while electrodes are formed to apply a modulating AC electric field to the ridge-shaped optical waveguide portion, it is theoretically known that as the angle of the ridge reaches 90.degree., the coefficient of correcting the electric field can be increased and the driving voltage can be lowered (JP-A-4 123 018). In JP-A-4 123 018, the coefficient of correcting the electric field was tried to be made as large as possible by setting the ratio d/w of the ridge-shaped optical wavelength at not less than 0.1 but not more than 1.0 and setting the angle of the ridge at 90.degree..+-.10.degree..
However, according to the above method, the following problem is still unsolved. That is, reactive ion etching, takes a very long time to uniformly etch the entire wafer having a diameter of, for example, 3 inches down to a depth of a few .mu.m, which results in costly working. Further, since high energy ions are irradiated upon the substrate, there occur problems that the substrate is likely to be damaged, a working denatured layer is formed at an optical waveguide through which essential light passes, and consequently characteristics such as refractive index changes. Since the formation of such a working denatured layer is not considered in simulating the optical waveguide device, actual characteristics of the optical waveguide device differ from those grasped in the simulation, which result in a cause for deterioration.
Furthermore, there is a limit to improvement upon the electric field correcting coefficient in the case of the above prior art light modulator. That is, an upper surface of the ridge-shaped optical waveguide portion is almost flat, its side faces are inclined, and a film of a modulating electrode is formed over these inclined side faces and a main plane of the epitaxial film. Therefore, as theoretically investigated in a literature OQE 77-57, "Ridge-shaped waveguide portions" published by Corporate Juridical Person: Electronic Communication Academy, Oct. 24, 1997, if the modulating AC electric field is applied to the optical waveguide, the modulating efficiency due to this alternative electric field decreases and the driving voltage is accordingly lower as compared with the ridge angle being 90.degree..
The reason why the side faces of the ridge-shaped optical waveguide portion are inclined is considered as follows. That is, the ridge-shaped optical waveguide portion is projected at the main plane of the epitaxial film, and at that time surrounding portions of such a ridge-shaped optical waveguide portion need to be etched as deep as possible so as to increase the ratio d/W in which d and W are the height and the width of the ridge-shaped optical waveguide, respectively, namely so as to project the ridge-shaped optical waveguide in a slender form. However, the ratio in the etching speed between the substrate and the mask is ordinarily 2:1 to 5:1. Therefore, in order to deeply etch the surrounding portions around the ridge-shaped optical waveguide, a mask having a correspondingly increased depth needs to be used. However, when such a thick mask is used, the ridge angle 0 decreases far less than 90.degree. because the etching rate decreases around the mask. For example, if the height W of the ridge-shaped optical waveguide is increased to 2 .mu.m or more, it is difficult to set the ridge angle to near 90.degree..